Technical Field
This invention relates to the tapping of steel from a steelmaking furnace such as an electric arc furnace (EAF) into a receiving vessel such as a ladle. More particularly, a method and apparatus for filling the ladle to a target level instead of to a target weight are disclosed. The method and apparatus enable a larger average tap weight per heat and therefore higher plant production rate.
Description of Related Art
In EAF steel production, the liquid steel is covered by primary steelmaking slag, mostly composed of oxides of metals, including calcium, silicon, iron, magnesium, aluminum, and manganese oxides. This primary slag is high in iron oxide content and is “oxidizing” with respect to acceptable dissolved oxygen content in steel prior to casting. Therefore, it is not suitable for further steel refining operations. During pouring of the molten steel, it is preferable not to let this slag flow from the furnace into the ladle, since it is usually undesirable in the next step of steel refining.
Historically, steel was commonly tapped from an electric arc furnace through a tapping spout in the side of the furnace. This method resulted in substantial quantities of slag pouring from the furnace along with the steel. U.S. Pat. No. 4,592,067 of Fuchs et al. discloses an apparatus designed to tap a furnace substantially “slag free” by replacing the tapping spout with a bottom tap hole, eccentrically placed. This and other similar arrangements have become common in the art. The eccentric bottom tap hole, or “EBT” as it is known in the art, has reduced the average amount of slag carried into the ladle from the furnace during tapping. For the EBT to function properly, a percentage of steel must be held inside the furnace to support the slag above the tap hole and prevent it from entering the ladle. This steel is called the “hot heel” and may be approximately 5% to 50% of the total charge, depending on the process. The approximate yield of liquid steel is typically known and the scrap charge weights are calculated to yield an average tap weight of steel while maintaining the desired hot heel amount for the next batch of steel. In the art, each batch of steel is commonly referred to as a “heat.”
The ladle that receives the steel from the EBT furnace is lined with refractory oxide material. This material wears away through erosion and chemical reactions, so a ladle has limited lifespan before it requires a new refractory lining. Typically, about 70-85% of the thickness of the refractory lining is worn away during the course of the useful life of that ladle lining, after which new refractory oxide is once again deployed in the ladle to start a new lining campaign, i.e., a series of heats performed over the lifetime of that ladle refractory lining. The volume of the ladle therefore increases with each heat tapped into it from the first heat to the last in the lining campaign. Since each heat is tapped to a constant target weight, the level of steel in the ladle decreases as the ladle lining wears away. It is not uncommon for the level of steel in the ladle to vary by approximately two feet or more during the course of a ladle lining campaign.
There are several disadvantages to the current method in the art of tapping steel to a consistent weight. One disadvantage is that tapping to a consistent weight results in variability of the level of slag that floats on the molten steel in the ladle. There is always slag disposed on top of the steel in the ladle, and this level range in the ladle refractory lining is referred to as the slag line. The refractory lining material must be of high quality at the slag line to resist the chemical attack by the slag. This material is often more costly than the other refractory material in the remainder of the ladle. The variable slag line position requires a large area of slag line refractory material. Less slag line-compatible refractory would be needed, and thus a lower cost of refractory materials would be incurred if the steel level and associated slag line were consistent within the ladle.
Another disadvantage of the variable steel level associated with tapping to a consistent weight is that the electrodes of the ladle metallurgy furnace (LMF) must be long enough and have large range of motion in the vertical direction to accommodate the variable steel level. This necessitates longer graphite electrodes, and a higher oxidation rate of the graphite. Thus a higher electrode cost per ton of steel refined is incurred.
There is also a practical limit to the level of steel that may occupy the ladle volume. Some steel refining processes are carried out in the ladle, causing turbulence and splashing of steel and slag. It is desirable to retain the steel and slag within the ladle. In vacuum tank degassing, for example, the process causes large wave formation and surface turbulence. The vacuum causes gas bubbles to form in the steel, increasing its volume. Therefore, the steel level must be kept a meter or more from the top of the ladle to ensure containment of the ladle contents. The distance between the top of the ladle and the surface of the contents of the ladle is referred to as the “freeboard.” If vacuum degassing is not required, but the heat will be treated with bottom gas stirring and arc heating, then a freeboard of about ½ meter may be appropriate. An even smaller freeboard may be used if the steel will not be subjected to secondary refining. Overall, the freeboard requirement defines a practical limit for filling the ladle.
Another aspect that can limit the level to which a ladle can be filled is the ladle lifting crane capacity. Molten steel is considerably more dense than refractory lining material, and thus a ladle that is filled to a target freeboard level and that has had a large amount of the refractory eroded away and its volume replaced with molten steel is much heavier that the same ladle filled to the same target freeboard level, but with a new and thicker refractory liner. It is conceivable that a ladle near the end of its useful lining life filled with steel to the target freeboard level can exceed a crane's safe lifting capacity. In such cases, the target freeboard level must be reduced commensurate with the applicable weight limitations.
For example, consider a typical steelmaking ladle with design capacity of 150 tons and new working lining refractory brick thickness of 8 inches. After 100 heats, the working lining thickness has been reduced to 2 inches and further use of the ladle is considered unsafe. The target weight of 150 tons of steel is matched with the target freeboard of the new ladle. The last heat on that ladle may have an additional 40 tons of capacity of molten steel to reach that target freeboard. For the sake of simplicity, if it is stipulated that each heat of steel generates equal excess capacity in the ladle, then the average steel weight in the ladle can be increased by 20 tons, or approximately 13%. A prerequisite for this scenario is adequate crane capacity to support the added weight. At present, to the best of the Applicants' knowledge, there is no method practiced in the art in which the steel is tapped to a consistent freeboard while anticipating the tap weight in order to make appropriate corrections to charge weight and alloy additions at tap.
What is needed is a method of tapping an electric arc furnace that maximizes yield and throughput of steel while reducing costs of the overall steel refining process.